Defect Properties of Alkaline-Earth Hydrides for Hydrogen Energy Applications

Heavy alkaline-earth hydrides (AeH₂; Ae = Ca, Sr, Ba) are thermally stable materials that have demonstrated good conductivity of hydride ions over a broad temperature range. However, experimental studies have not conclusively determined the dominant mechanisms for hydride ion transport in these materials. To better understand these mechanisms and, in the process, move these materials closer to applications, we conduct first-principles calculations based on density functional theory with a hybrid functional. We characterize the bulk electronic and structural properties of CaH₂, SrH₂, and BaH₂, focusing specifically on the correlation between native point defect properties and high ionic conductivity. We calculate low migration barriers for hydrogen vacancy-mediated conductivity, particularly for vacancies in the positive charge state; this suggests processing conditions should aim to increase the concentration of positively charged hydrogen vacancies. We show that doping these hydrides with certain alkali metals can lower the formation energies of such defects, justifying experimental reports of improved conductivity in doped alkaline-earth hydrides and guiding development of materials with superior properties.